Connecting an Underwater Vehicle to a Tether

ABSTRACT

The connection of an underwater vehicle by a tether and apparatus for such connection is disclosed. The apparatus comprises a drum which has a helical surface and which stores the tether so that the tether is unwound from the drum to allow the underwater vehicle to move away from the apparatus. The apparatus also includes a pulley system which engages with the tether and which also unwinds the tether from the drum. The apparatus further includes a pulley-drive and a drum-drive. The pulley-drive is operable in a forward mode to drive the pulley system for the unwinding of the tether and operable in a braking mode to resist the re-winding of the tether and the drum-drive is operable in a forward mode to drive the drum for the re-winding of the tether and operable in a braking mode to resist the unwinding of the tether.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from United Kingdom Patent ApplicationNo. 14 03 694.1, filed 1 Mar. 2014, the entire disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for connection to anunderwater vehicle by a tether. The present invention also relates to amethod of unwinding a tether connected to an underwater vehicle from anunderwater drum to allow movement of the vehicle. The present inventionalso relates to a method of re-winding a tether connected to aunderwater vehicle onto an underwater drum.

2. Description of the Related Art

It is known to use a tether management system (TMS) in which a buoyanttether is winched onto a submersed drum, so that an underwater vehiclemay travel freely around the tether management system, performingrequired duties without being subject to movements occurring to asupport vessel and without requiring the support vessel to move.

Given that the tether is buoyant in water, the underwater vehicle doesnot apply tension to the tether, therefore the spooling of the tethermay become difficult. In known systems, additional mechanisms aredeployed to position the tether upon a particular region of the cabledrum. However, these mechanisms are subject to failure and a degree ofexpertise is required during a reconnection process.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedan apparatus for connection to an underwater vehicle by a tether,comprising: a drum having a helical surface and which is configured tostore said tether such that said tether is unwound from said drum toallow said vehicle to move away from the apparatus; a pulley systemconfigured to engage said tether and unwind the tether from said drum; apulley-drive operable in a forward mode to drive said pulley system forthe unwinding of the tether and operable in a braking mode to resist there-winding of said tether; and a drum-drive operable in a forward modeto drive said drum for the re-winding of said tether and operable in abraking mode to resist the unwinding of the tether.

In an embodiment, the tether conveys power, control signals andcommunication signals.

According to a second aspect of the present invention, there is provideda method of unwinding a tether connected to an underwater vehicle, froman underwater drum to allow movement of said vehicle, said drum having acontinuous groove defining a substantially helical surface, said methodcomprising the steps of: driving a pulley system configured to engagewith said tether to unwind said tether from said drum; and activatingsaid drum to maintain transported tether between the drum and saidpulley system in tension.

In an embodiment, the pulley system includes a primary pulley defining aradius of curvature for the tether and configured to engage on aninternal surface of the tether; and a plurality of support pulleysconfigured to engage with an external surface of the tether inopposition to said primary pulley.

According to a third aspect of the present invention, there is provideda method of re-winding a tether connected to an underwater vehicle, ontoan underwater drum, said drum having a continuous groove defining asubstantially helical surface, said method comprising the steps of:driving said drum to rewind said tether onto the drum; and activating apulley system configured to engage with the tether to maintaintransported tether between the pulley system and the drum in tension.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a tether management system embodying the present invention;

FIG. 2 shows the tether connected to an underwater vehicle;

FIG. 3 details the tether management apparatus;

FIG. 4 shows a cross-section of the tether management apparatus;

FIG. 5 shows a hydraulic circuit; and

FIG. 6 details a portion of a pulley in contact with a tether.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1

An apparatus 101 is shown in FIG. 1, and may be identified as a tethermanagement system (TMS) for tethering an underwater vehicle 102,possibly taking the form of a remotely operated vehicle receivingcontrol signals from a control station within a support vessel 103.

In the configuration shown in FIG. 1, the tether management system 101appears as if sitting on top of the underwater vehicle 102 and as suchis sometimes referred to as a “tophat”. The support vessel 103 includesa launch and recovery system 104, including a drum 105, a swinging frame106 and an umbilical cable 107. A winch is configured to lift and loadapparatus 101 off and on the support vessel 103 and in use the apparatusmay operate at depths of up to four kilometre (4 km).

Within the apparatus 101, a drum 108 is configured to store a tether109, such that the tether 109 is unwound from the drum 108 to allow thevehicle 102 to move away from the apparatus. The vehicle 102 may travelin any direction away from the apparatus 101, typically up to a distanceof one thousand five hundred metre (1500 m).

FIG. 2

As illustrated in FIG. 2, the support apparatus 101 is connected to thesupport vessel 103 by an umbilical cable 107. The weight of apparatus101, even when submerged, is sufficient to maintain the umbilical cable107 in tension, thereby facilitating the spooling and unspooling of theumbilical cable 107 onto and off from the drum 105.

The tether management drum 108 has been activated thereby allowingtether 109 to be removed, such that the underwater vehicle 102 may moveaway from the tether management apparatus 101.

Remotely operated underwater vehicles, such as vehicle 102, are oftenbuilt with a large floatation pack on top of an aluminium chassis toprovide buoyancy while tasks are being performed. A tooling skid may befitted to the bottom of the vehicle to accommodate a variety of sensorsand tooling packages. Light components tend to be raised towards the topof the vehicle, with heavy components being at the bottom. Electricalcomponents are contained within oil-filled watertight containers toprotect them from water corrosion and water pressure.

In the oil industry, crews typically work two twelve-hour shifts and thevehicle 102 itself may be underwater for most of this time. Failure ofthe tether management apparatus 101 is highly undesirable, given thatfacilities for effecting repairs at sea are limited and system downtimesincur significant costs. It is therefore highly undesirable for problemsto occur due to the mis-spooling of tether 109 when the underwatervehicle 102 is being brought back to the tether management apparatus 101at the end of a shift.

As shown in FIG. 2, the re-winding of tether 109 is made problematic dueits buoyancy. Unlike umbilical cable 107, the tether 109 will often beslack, due to the vehicle 102 going out to an extreme distance and thenretracting back towards the tether management apparatus. Thus, thisslack must be taken up before the tether 109 is again placed in tension.This problem does not occur with the umbilical cable 107 because thisalways remains substantially in tension due to the weight of the tethermanagement apparatus 101.

The tether 109 is not merely a link between the tether managementapparatus 101 and the vehicle 102. The umbilical 107 provides a voltageof anything up to four kilovolts (4 kv) and experience suggests that avoltage of one kilovolt per kilometre (1 kv/km) of cable is required inorder to guarantee the availability of power at the tether managementsystem 101 and the remote vehicle 102.

The tether also conveys control signals to the vehicle 102, relayed viathe umbilical 107 and the tether management apparatus 101 and returnsdata and images back to the support vessel 103. Thus, the tether 109represents a complex assembly of materials but given the requirement forbuoyancy, the tether 109 tends to be less robust than the umbilicalcable 107. Furthermore, it is undesirable for excessive unnecessarylengths of tether to be released, given that this will cause additionaldrag when underwater currents are present.

In many applications, it is not possible to use a free swimmingunderwater vehicle connected directly to the support vessel 103. Thetether management system allows better access to plant without bringingthe support vessel 103 too close. Furthermore, in some installations aminimum distance must be maintained between vessels and operationalplant.

Furthermore, the tether management system reduces the extent to whichthe support vessel 103 is required to move. Its presence also isolatesthe remote vehicle 102 from natural movements of the support vessel 103,possibly due to swell.

FIG. 3

A frame 301 of the tether management system 101 is illustrated in FIG.3. Tether drum 302 has been located within the frame 301. In thisembodiment, drum 302 has a helical surface 303. In this embodiment, thehelical surface is provided by a helical sleeve applied over asubstantially smooth drum.

FIG. 4

The management system 101 identified in FIG. 1 is detailed in FIG. 4. Aspreviously described, a frame 301 supports a tether drum 302. The tetherdrum 302 supports a tether such that the apparatus may be connected toan underwater vehicle as shown in FIG. 2. The tether drum 302 stores thetether 109 such that the tether 109 may be unwound from the tether drum302 to allow an underwater vehicle (such as vehicle 102) to move awayfrom the apparatus.

A pulley system 303 is configured to engage with the tether 109 so as toallow the tether to be unwound from the drum. A pulley-drive isprovided, for driving the pulley system 303, in a forward mode to drivethe pulley system for the unwinding of the tether thus moving the tetherin the direction of arrow 401 and ultimately allowing the tether to exitthe apparatus from an exit port 402. In addition, the pulley-drive isoperable in a breaking mode to resist the re-winding of the tether.

A drum drive is operable in a forward mode to drive the tether drum 302for the re-winding of the tether, thereby moving the tether in thedirection of arrow 402. Furthermore, the drum-drive is also operable ina breaking mode to resist the unwinding of the tether. Thus, when thetether 301 moves in the direction of arrow 401, the pulley-drive isoperable in its forward mode, thereby pulling the tether away from thedrum. However, while this occurs, the drum 302 is in its breaking mode,applying force in the direction of arrow 402. The driving force of thepulley ensures that the tether does move in the direction of arrow 401,against the breaking force of the drum-drive. However, with these forcesoperating in opposite directions, the tether between the drum 302 andthe pulley system 303 remains in tension, thereby ensuring that thetether is removed from the drum in a smooth operation.

With the drum-drive operable in a forward mode, the tether moves in thedirection of arrow 402, thereby returning the tether to the drum. Thepulley system 303 operates in its breaking mode, applying a reduceddegree of force in the direction of arrow 401, thus resisting themovement of the drum 302. Drum 302 continues to move and the tethercontinues to be wound onto the drum. However, it is not possible for thetether to bunch or become slack, because the length of tether betweenthe drum 302 and the pulley system 303 remains in tension.

The pulley system 303 includes a primary pulley 403 defining a radius ofcurvature 404 for the tether 109 as it moves. This radius of curvatureis sufficient to ensure that the tether 109 does not experienceexcessive bending forces which may result in tether damage and failure.

A plurality of support pulleys are also provided which, in thisembodiment, include a first support pulley 405 and a second supportpulley 406. The support pulleys 405 and 406 are configured to engagewith an external surface 407 of the tether 109 in opposition to theprimary pulley 403. Thus, in operation, the tether 109 is held securelybetween the primary pulley 403 and the secondary pulleys 405, 406. Thisoverall region of support may be referred to as a sheave and, given itsdriven nature, it may collectively be referred to as a power sheave.

The apparatus 101 provides for the tether to exit from exit port 402 ina substantially vertical direction. A flexible support conduit 408supports the tether as it passes between the pulley system and the exitport 402. In an embodiment, the conduit 408 is fabricated from aplurality of interlocking portions, thereby facilitating flexibilityduring assembly and maintenance procedures.

FIG. 5

In an embodiment, pulley-drive 403 is driven by a hydraulic motor 501.Similarly, drum 302 is driven by a second hydraulic motor 502. Anelectric motor receives electrical power from the umbilical 107 and usesthis to apply pressure to a hydraulic fluid circulated within ahydraulic circuit; a schematic version of which is shown in FIG. 5.

Hydraulic fluid from the hydraulic pump is supplied to a valve 503 froma main inlet 504. Valve 503 may receive a first pilot signal on an inputline 505, resulting in pressure being supplied to line 506. The valve503 may also receive a pilot signal on input line 507 resulting inpressure being applied to output line 508. The valve 503 may also remainin a rest condition, resulting in neither line 506 nor line 508 beingenergised.

A pilot signal on line 505 represents a spool out condition, forcing thetether out of orifice 401 through movement in the direction of arrow401. A valve 509 directs high pressure fluid to the pulley-drive 501causing the pulley-drive to pull tether away from drum 302. In addition,a further valve 510 supplies a lower breaking pressure to the drum-drive502. Thus, the force applied to the pulley-drive 501 overcomes the forceapplied to the drum-drive 502 but with a positive pressure being appliedto the drum-drive 502; the length of tether held between the drum 302and the pulley system 403 remains in tension.

The application of a pilot signal to input line 507 results in pressurebeing applied to line 508. This in turn supplies driving pressure todrum-drive 502 via a valve 511. However, in addition, when thedrum-drive receives a drive pressure, the pulley-drive 501 receives alower breaking pressure via a valve 512. In this mode, the main drivepressure supplied to the drum-drive 502 results in a re-winding of thetether, with movement of the tether in the direction of arrow 402.However, given the breaking pressure supplied to the pulley-drive 501,via valve 512, the tether 109 between the pulley 403 and the drum 302remains in tension. With the application of this tension, the tether 109is forced to re-wind onto the drum 302 as intended, without thepossibility of bunching or disruption.

Thus, the hydraulic system of FIG. 5 allows the apparatus to be deployedin accordance with two methods of operation. During an unwinding method,a tether is connected to a underwater vehicle from an underwater drum302 to allow movement of the vehicle. A pulley system 403 is driven,configured to engage with the tether so as to unwind the tether from thedrum. Furthermore, the drum is also activated (at lower pressure) so asto maintain transported tether 109 between the drum and the pulleysystem in tension.

A second method of operation is provided for the re-winding of thetether, with the tether connected to an underwater vehicle. The tetherre-winds onto underwater drum 302. The drum is driven so as to re-windthe tether onto the drum. Furthermore, pulley system 403 is alsoactivated so as to engage with the tether, to maintain transportedtether 109 between the pulley system 103 and the drum 302 in tension.

As shown in FIG. 3, in an embodiment, the drum 302 has a continuousgroove defining a substantially helical surface. In this way, a firstlayer of the rewound tether may be forced into this groove, due to theactivation of the pulley system maintaining the tether in tension. Afterthe first layer of the rewound cable has been positioned, due to thepresence of the helical groove, a second layer may be rewound so as tofall within grooves defined by the first layer. Thus, this process maycontinue until all of the tether has been rewound onto the tether drum302.

In an embodiment, the underwater vehicle is then restrained by a springloaded device when the tether has been fully rewound. In this way, it ispossible for the underwater vehicle to be restrained in the absence ofpower but for the vehicle to be released, activation of the springloaded device is required. In an embodiment, hydraulic pressure is usedto effect this release in response to the spool-out pilot signal beinggenerated.

FIG. 6

An example of pulley 403 is illustrated in FIG. 6. This primary pulleydefines a groove 601 configured to make contact with tether 109 suchthat force may be applied in the direction of arrow 401 in order toremove tether from cable 303. In an embodiment, the pulley systemeffectively maintains a tight grip on the tether 109, given that thetether is being maintained in tension by the activation pressure appliedto drum-drive 502.

In an embodiment, the profile of groove 601 is such that as the diameterof the tether 109 reduces, the tether is forced radially inwards, suchthat it is still held by the reducing width of groove 601. Furthermore,in an embodiment, pulley 403 is cast from polymer material, so as toenhance the frictional force between groove 601 and tether 109.

What we claim is:
 1. Apparatus for connection to an underwater vehicleby a tether, comprising: a drum having a helical surface and which isconfigured to store said tether such that said tether is unwound fromsaid drum to allow said vehicle to move away from the apparatus; apulley system configured to engage with said tether and unwind thetether from said drum; a pulley-drive operable in a forward mode todrive said pulley system for the unwinding of the tether and operable ina braking mode to resist the re-winding of said tether; and a drum-driveoperable in a forward mode to drive said drum for the re-winding of saidtether and operable in a braking mode to resist the unwinding of thetether.
 2. The apparatus of claim 1, wherein said tether conveys power,control signals and communication signals.
 3. The apparatus of claim 2,connectable to a surface device by an umbilical cable.
 4. The apparatusof claim 3, wherein said power and said control signal are received fromsaid umbilical cable.
 5. The apparatus of claim 1, connectable to asurface device by an umbilical cable.
 6. The apparatus of claim 4,wherein said underwater vehicle is a remotely controlled vehicleconfigured to receive control signal from said surface device.
 7. Theapparatus of claim 1, wherein said tether is buoyant in water.
 8. Theapparatus of claim 1, wherein said helical surface is provided by ahelical sleeve applied over a substantially smooth drum.
 9. Theapparatus of claim 1, wherein said pulley-drive includes a firsthydraulic motor and said drum drive includes a second hydraulic motor.10. The apparatus of claim 9, wherein: said pulley-drive receives adrive pressure and said drum-drive receives a lower braking pressurewhen unwinding the tether; and said drum-drive receives a drive pressureand said pulley-drive receives a lower braking pressure when rewindingsaid tether.
 11. A method of unwinding a tether connected to anunderwater vehicle, from an underwater drum to allow movement of saidvehicle, said drum having a continuous groove defining a substantiallyhelical surface, said method comprising the steps of: driving a pulleysystem configured to engage with said tether so as to unwind said tetherfrom said drum; and activating said drum so as to maintain transportedtether between the drum and said pulley system in tension.
 12. Themethod of claim 11, wherein said pulley system includes: a primarypulley defining a radius of curvature for the tether and configured toengage on an internal surface of the tether; and a plurality of supportpulleys configured to engage with an external surface of the tether inopposition to said primary pulley.
 13. The method of claim 11, whereinsaid primary pulley defines a groove and said groove is configured tomaintain contact with the tether during tether shrinkage at depth. 14.The method of claim 11, including an exit port for allowing theunwinding of the tether from the apparatus in a substantially verticaldirection.
 15. The method of claim 14, including a flexible supportconduit for supporting the tether between the pulley system and saidexit port.
 16. A method of rewinding a tether connected to an underwatervehicle, onto an underwater drum, said drum having a continuous groovedefining a substantially helical surface, said method comprising thesteps of: driving said drum so as to rewind said tether onto the drum;and activating a pulley system configured to engage with the tether soas to maintain transported tether between the pulley system and the drumin tension.
 17. The method of claim 16, wherein a first layer of rewoundtether is forced into said groove due to the activation of said pulleysystem maintaining the tether in tension.
 18. The method of claim 17,wherein said first layer of rewound cable defines a secondary helicalgroove and a second layer of rewound tether is forced into saidsecondary groove.
 19. The method of claim 16, wherein said underwatervehicle is restrained by a spring-loaded device when the tether has beenfully re-wound.
 20. The method of claim 18, wherein said underwatervehicle is restrained by a spring-loaded device when the tether has beenfully re-wound.